The media knows that most of their readers/listeners/viewers have short attention spans. It reminds me of the same scenario that played out earlier this year, last March in fact, when the US Military announced an ongoing investigation into abuses at the Abu Ghraib prison near Baghdad. The media covered it then, it became just another story out of Iraq, lasted one news cycle. Seemingly, a few short months later, Kerry's campaign is in trouble, the story bubbled up from beneath the surface as some "new revelation" and the media ran with it to bring down the Bush administration because Kerry couldn't chip away at Bush's strong defense policies. So the media felt they had to step in and help, and dredged up this nonstarter of a story and ran with it until the American public realized the real intention of the story.
Of course, the media didn't get much help from Nicholas Berg's whacked out anti-war father. Both the Berg beheading, as gruesome and unfortunate as it was, and the dredging up of the Abu Ghraib prison scandal story happend at precisely the same time. Kerry cannot campaign around, so he's being helped around by a bunch of hard left media whores, like the drunk prom date.